The biochemical toxicity, histological changes and mechanism of lung injury by cyclophosphamide (CP) and its postulated toxic metabolites, acrolein and phosphoramide mustard, will be studied in rats. Emphasis will be placed on three potential mechanisms of CP-induced lung toxicity involving metabolic activation: I. The ability of the lung to metabolize CP to its reactive metabolites will be investigated in vitro, in vivo, and in an isolated, perfused lung system using ring (4-C14)-CP. In vitro we will incubate CP with 9,000xg, cytosolic and microsomal lung fractions to identify reactive metabolites formed. Blooc levels of CP and its reactive metabolites, as well as covalent binding of CP and its reactive metabolites to lung homogenates, mitochondria, cytosol and microsomal fractions, will be studied in the lung of animals treated with ring (4-C14)-CP. In isolated, perfused studies, the identification of reactive metabolites in the perfusate and assessment of covalent binding to tissue fractions will be attempted; II. The role of the liver in forming reactive metabolites and then transporting them to the lung via the circulation will be investigated by monitoring blood levels of metabolites with and without altering the rate of CP metabolism in liver by known metabolic inducers and inhibitors; III. The role of reactive metabolites of CP in free-radical formation will be studied by assaying the lipid peroxide formation and the activities of the lung antioxidant enzymes. The effect of oxygen and CP treatment on free-radical formation and the protective effect of the antioxidants, vitamins E and C, will be established. The nature of CP metabolism-dependent alteration of microsomal mixed-function oxidase (MFO) activities of NADPH-cytocrome c reductase, aniline hydroxylase, benzphetamine N-demethylase and NADPH-dependent lipid peroxide formation as well as total glutathione (GSH) content and antioxidant enzyme activities will be measured in the tissue and subcellular fractions used in parts I, II and III, as parameters of biochemical damage. The effect of isolated reactive metabolites of CP on the histological changes and the above biochemical parameters will be assessed and compared to the parent compound CP. Finally, information obtained form parts I, II and III will be precisely correlated with biochemical damage and histological changes in order to establish the mechanism of CP-induced lung injury.